1. Field of the Invention
This invention relates to an improvement in the treatment of high strength liquors and wastewaters by anaerobic digestion for the removal of organic matter and color.
2. Information Disclosure Statement
Anaerobic digestion of sewage sludges has been commonly practiced for many years. In this treatment process, organic matter in waste sludges is converted primarily to methane and carbon dioxide in an oxygen-free environment.
When compared to aerobic treatment processes, anaerobic digestion has several disadvantages. First, anaerobic digestion generally results in a considerably lower degree of organic matter removal. Secondly, the activity of anaerobic bacteria has proven to be very sensitive to pH, temperature, and toxic materials such as ammonia, pyridine, and heavy metals. The methanogenic bacteria reproduce at a low rate, so full recovery of a digester following an upset condition usually takes several weeks, and often several months.
From a theoretical standpoint, however, the anaerobic treatment process is more energy efficient than aerobic processes, since (a) transfer of large quantities of air or oxygen to the waste is not required, (b) a significant portion of the organic matter is converted to methane, a useful combustible fuel gas, and (c) anaerobic treatment results in a markedly lower production of new bacterial cells, biomass that must ultimately be disposed of.
Anaerobic treatment processes have required longer treatment residence times than aerobic processes. Anaerobic digestion of sewage sludges for 10-30 days is commonly practiced.
Thermal treatment of sludges, i.e., wet oxidation or "heat treatment", produces high strength liquors which are often highly colored. Neither aerobic or anaerobic treatment results in significant color removal from such high strength liquors; the color level may actually increase as a result of biological treatment.
Anaerobic digestion of sewage sludge with the addition of powdered activated carbon has been previously attempted. A paper by R. R. Spencer, A. J. Shukrow and J. F. Ferguson entitled "The Addition of Powdered Activated Carbon to Anaerobic Digesters: Effects on Methane Production", Battelle Laboratories Report No. BNWL-SA-5893, July, 1976 noted an improvement in gas production from the addition of carbon. However, destruction of sludge materials and production of gas were unacceptable at all but the highest carbon concentrations and longest residence times. At 2.5 days residence time, all tests were unacceptable.
Statham U.S. Pat. No. 2,059,286 discloses the use of small amounts (about 3-10 ppm) of activated carbon to sewage sludge undergoing anaerobic digestion. The patent claims that digestion time was thus reduced from six months to about three months.
Alan D. Adams, "Activated Carbon: Old Solution to Old Problem", Water & Sewage Works, August, 1975, pp 46-48; and September, 1975, pp 78-80, reported on crude initial tests by ICI America wherein activated carbon was added to anaerobic digesters. Mixed results were reported.
Addition of powdered activated carbon to an overloaded, poorly-operated two-stage digestion system was reported by M. Hunsicker et al., "Powdered Activated Carbon Improves Anaerobic Digestion", Water and Sewage Works, pp 62-63, July, 1976. Improvements in odor, sludge settling and gas production were noted.
Further tests were reported in Carl M. Koch et al., "Wanted: More Gas, Less Sludge--Can PAC Help?", Water and Wastes Engineering, pp. 27-29, June, 1978. The authors concluded that the use of activated carbon in sludge digesters does not enhance the digestion process unless the digesters are overloaded and operating poorly.
Anaerobic treatment of wastewater with a fluidized bed of particle support media (plastic, sand, coal, or activated carbon) in the size range of 0.2 to 0.3 mm was disclosed by Jeris, U.S. Pat. No. 4,182,675 and Barnes et al., U.S. Pat. No. 4,505,819. Wastewater feed BOD.sub.5 ranged from 50 to 50,000 mg/l. Both inventions require substantial energy to suspend the relatively large particles in the fluidized bed. The anaerobic digester of Rovel et al., U.S. Pat. No. 4,482,458 contains similar granular support media of less than 1 mm size. Recirculation of digester contents is accomplished by pressurizing the methane-CO.sub.2 biogas to a high overpressure and injecting the gas into a draft tube within the digester. This also requires considerable energy.
In Japanese Patent Publication No. 62,357/78, published June 3, 1978, of Niigata Engineering Co., Ltd., high-strength wastewater is diluted, mixed with activated carbon and passed through four biological stages comprising (a) anaerobic denitrification, (b) aerobic nitrification and organic carbon removal, (c) a second anaerobic denitrification and (d) a final aerobic treatment step. Fuel gas is not produced because virtually no organic matter destruction occurs in the anaerobic denitrification stages.
J. Willard DeBauche, Jr., "Research Report, Green Bay Metropolitan Sewerage District 1980-82 Anaerobic Digestion of Heat Treatment Liquor" described an anaerobic contact suspended growth system operating without added activated carbon but with 2.56 g/l total suspended solids comprised solely of biomass. The reported reduction in liquor BOD.sub.5 was 56 percent.